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Attempt to Control the Invasive Red Alga Acanthophora spicifera 

(Rhodophyta: Ceramiales) in a Hawaiian Fishpond: An Assessment 

of Removal Techniques and Management Options 1 

Mariska Weijemtan, 2 ' 3, Rebecca Most, 1, Kristy Wong, 1, and Sallie Beavers 1, 

Abstract: Acanthophora spicifera (Vahl) Borgesen was unintentionally introduced 
to Hawai'i in 1950 and has since become the most common nonindigenous 
algal species in the main Hawaiian Islands. On the west coast of Hawai'i Island 
it has been documented at three sites, including Kaloko Fishpond in Kaloko- 
Honokohau National Historical Park. The fishpond has an open connection to 
the sea, increasing the risk that A. spicifera will establish itself on neighboring 
shallow coral reefs and rocky intertidal habitats. To diminish that risk and to de- 
velop an efficient management strategy, a range of approaches was assessed to 
control this invasive alga in Kaloko Fishpond. Removal techniques were la- 
bor intensive and had limited effect. All experiments showed a substantial initial 
decrease in algal density, but the long-term effect was minimal because of rapid 
regrowth. The most promising removal method was the use of submerged shel- 
ters to raise local densities of herbivorous fishes. Fishes grazed the alga and 
quickly reduced the biomass. However, the large number of predators and ab- 
sence of topographical structure will make it challenging to provide sufficient 
shelters to increase the herbivorous fish population in the entire fishpond. A 
management strategy to substantially reduce the algal biomass in the fishpond 
includes a combination of biological control and periodic manual removal of 
the alga. 



Numerous publications have docu- 
mented dramatic changes in marine ecosys- 
tems after invasive species intentionally or 
unintentionally were moved across geograph- 
ical or ecological barriers (Carlton 1987, 
Boudouresque et al. 1995, Trowbridge 1995, 
Jousson et al. 2000, Curiel et al. 2001, Smith 
et al. 2002, 2004, Conklin and Smith 2005). 
In response to the seriousness of invasive 
species issues, state, federal, and nongovern- 



1 This study was funded by the National Park Service 
Water Resource Division through the Pacific Cooperative 
Studies Unit, University of Hawai'i at Manoa, Honolulu. 
Manuscript accepted 26 October 2007. Use of product 
names does not imply endorsement by NPS or UH. 

2 Author for correspondence (e-mail: Mariska 
_Weijerman@contractor.nps.gov). 

3 Kaloko-Honokohau National Historical Park, 73- 
4786 Kanalani Street, No. 14, Kailua-Kona, Hawai'i 96740. 



Pacific Science (2008), vol. 62, no. 4:517-532 
Work of the U.S. Government 
Not under copyright 



mental agencies have developed programs 
or initiatives to determine the impacts of 
these species on ecosystem structure and to 
develop strategies to minimize those impacts. 
Management of invasives by implementing 
eradication or control programs, although 
difficult, can be successful, as seen in nu- 
merous terrestrial examples of invasive plant 
species (e.g., Foxcroft and Richardson 2003, 
Van Wilgen et al. 2004). Development of 
management options for control of marine 
invasives is much more challenging (e.g., 
Conklin and Smith 2005). Possibly the only 
presumed successful marine example is the 
short-term "injection" of bleach under a 
tarped-off area for Caulerpa taxifolia in a 
coastal lagoon of southern California (Wood- 
field and Merkel 2005). 

Hawai'i is a major recipient of introduced 
species. A conservative estimate of the num- 
ber of marine introduced species in Hawai'i 
is in the hundreds (Carlton 1987). There are 
19 documented species of introduced macro- 
algae in Hawai'i, at least five of which have 



517 



518 



PACIFIC SCIENCE ■ October 2008 



become invasive: Acanthophora spicifera, Av- 
rainvilka amaldelpha, Gracilaria salicornia, 
Hypnea musciformis, and Kappaphycus spp. 
(Rodgers and Cox 1999, Eldredge and Carl- 
ton 2002, Smith et al. 2002, 2004). To date, 
A. spicifera and Kappaphycus spp. are the only 
taxa that are known to be sexually reproduc- 
tive on a regular basis in the Hawaiian Islands 
(J. E. Smith, pers. comm.), thus increasing 
their dispersal potential. In Kane'ohe Bay, 
O'ahu, Kappaphycus spp. were introduced on 
the assumption that they would not spread 
(Russell 1983). However, surveys conducted 
in 1996 (Rodgers and Cox 1999), 1999 (Woo 
2000), and 2002 (Conklin and Smith 2005) 
found that the algae had spread as far as 6 
km away from the introduction site and were 
continuing to spread northward in the bay, 
where they were overgrowing live coral. 
Once invasive macroalgae have become es- 
tablished, eradication is difficult and costly, 
and can lead to high economic losses (Cesar 
and Beukering 2004). 

Acanthophora spicifera (Vahl) Borgesen, 
1910, a marine red alga, arrived in Pearl Har- 
bor, O'ahu, from Guam on a barge in the 
1950s (Russell 1992). Now it is the most 
common nonindigenous algal species in the 
state and displaces many native species where 
it is abundant (Smith et al. 2002). The success 
of A. spicifera in invading benthic habitats is 
attributed to: (1) its ability to reproduce both 
sexually and vegetatively (by fragmentation); 
(2) successful epiphytism; and (3) its adapt- 
ability to a wide range of hydrological con- 
ditions (Russell 1992). When and how A. 
spicifera first entered Kaloko Fishpond is un- 
known. Despite its dominance on other is- 
lands, on the west coast of Hawai'i Island 
its presence has been documented at only 
three sites: Pu'ukohola Heiau National His- 
toric Site (Ball 1977; L. Basch, unpubl. data, 
2005), Pu'uhonua o Honaunau National His- 
torical Park (C. Squair, unpubl. data, 2006), 
and Kaloko-Honokohau National Historical 
Park (Marine Research Consultants 2000, 
Smith et al. 2002). Only in Kaloko Fishpond, 
located in Kaloko-Honokohau National His- 
torical Park, has the alga been observed in 
abundance. The alga might have been present 
at low levels within the pond for much longer 



(beginning sometime after the species' wider 
introduction to the west coast of Hawai'i), 
but it apparently did not become invasive in 
Kaloko Fishpond until the late 1990s. The 
presence of this species was not mentioned 
in a 1971 evaluation of fishponds (Kikuchi 
and Belshe 1971), nor in a 1988 marine in- 
ventory study (Chai 1991) nor in a 1996 bio- 
logical and water quality study (Brock and 
Kam 1997). The first report of A. spicifera be- 
ing abundant throughout the fishpond was 
made in 2000 (Marine Research Consultants 
2000). Currently, it is the dominant macroal- 
gal species in the pond. 

In the last three decades, two important al- 
terations have changed the hydrology of the 
fishpond and might have influenced the sus- 
ceptibility of the pond to invasion by A. spici- 
fera. First, the condition of the fishpond wall 
separating the pond from the ocean has been 
continuously changing: It was left to deterio- 
rate from wave and wind action from the 
1970s to the late 1990s, at which time reha- 
bilitation efforts began (Bond and Gmirkin 
2003). Alterations in the integrity of the wall 
led to changes in water quality within the 
pond and the rate of exchange with adjacent 
ocean waters, and are therefore probably as- 
sociated with shifts in the pond biota. Second, 
mangroves that had invaded the pond periph- 
ery in the 1980s were removed in the early 
1990s (Bond and Gmirkin 2003). Accumula- 
tion of detritus in mangrove stands would 
have changed the bottom sediment and water 
quality, and likely also affected the pond bi- 
ota. In addition, upslope development begin- 
ning in earnest two decades ago has most 
likely led to nutrient increases in the ground- 
water that enters the fishpond. 

In the summer of 2003, the first compre- 
hensive study to quantify the distribution 
and density of A. spicifera in Kaloko Fishpond 
was conducted. A 20-m grid was laid out over 
the pond and at each grid point the presence 
and, if present, the abundance (from sparse to 
concentrated) of A. spicifera was scored. At 
that time, the pond bottom was 48% covered 
by A. spicifera either as drift, embedded in 
silt or sand, or attached to oysters or rock 
substratum (National Park Service, unpubl. 
data). A primary concern for Park resource 



Acanthophora spicifera Removal Attempts in a Hawaiian Fishpond ■ Weijerman et al. 



519 



managers was, and is, that A. spicifera might 
spread to the intertidal zone and coral reefs 
just seaward of Kaloko Fishpond. Fragments 
of drift A. spicifera have escaped the pond 
through the sluice channels (M.W., pers. 
obs.). The current absence of visible A. spici- 
fera on the adjacent reef could be because the 
alga is not able to establish outside the pond, 
perhaps due to locally high-grazing intensity, 
or because propagule pressure has so far been 
insufficient to promote establishment. How- 
ever, because of the prevailing surface and 
deep-water currents offshore of Kaloko Fish- 
pond (Storlazzi and Presto 2005), we cannot 
discount the potential for this alien alga to es- 
tablish on nearby shallow reefs. 

To reduce the risk of A. spicifera spreading 
to adjacent reefs, the following techniques 
were used from April to December 2006, to 
determine the feasibility of controlling or 
eradicating A. spicifera in Kaloko Fishpond: 
manual removal, shading of the benthos, 
and use of herbivorous fish (biocontrol). 
Chemical-control methods were not explored 
in this study because of the associated risks to 
aquatic organisms and ecosystems within this 
national park unit. It soon became apparent 
that A. spicifera regrew rapidly after manual 
removal. Therefore, trials to determine an ef- 
ficient manual removal technique were also 
conducted. These included cropping to three 
different specific thallus lengths (minimum 
viable thallus length trial), repeatedly remov- 
ing the same alga (repetitive removal trial), 
and removing attached algae with their hard 
substrate and replacing the substrate after 
a period of drying (substrate removal/ 
replacement trial). In each case we measured 
extent and speed of recovery of A. spicifera 
populations for periods of up to 4 months. 



MATERIALS AND METHODS 

Study Site 

Kaloko Fishpond is a 4. 5 -ha ancient Hawai- 
ian fishpond (Figure 1) located in Kaloko- 
Honokohau National Historical Park on the 
west coast of Hawai'i Island. It is a shallow 
(maximum depth 3.5 m, mean depth 1.5 m) 
natural embayment with a human-made dry- 



stack stone wall partially closing it off from 
the ocean. Two sluice channels in the wall al- 
low some water circulation between the ocean 
and pond. The fishpond seawall deteriorated 
substantially from about 1970 until rehabili- 
tation began in 1998 because of the impact 
of winter storm swells (Bond and Gmirkin 
2003). At the time of the study about 60% 
of the wall and one sluice channel were re- 
habilitated. Work on the wall continued 
throughout the study. Kaloko Fishpond water 
is stratified due to freshwater springs and 
seepage from cracks in the basalt bottom. 
The dominant substrate is silt composed of 
decomposing organic material varying in 
depth from a few centimeters on the ocean 
side of the pond to at least 1 m on the inland 
side, often with anoxic conditions (Kikuchi 
and Belshe 1971). Allochthonous sand is pre- 
dominantly found on the ocean side of the 
pond. The remaining bottom type is hard 
substrate such as basalt rock and oyster con- 
glomerates with an associated biota that 
includes anemones, fireworms, sponges, tuni- 
ca tes, and brittle stars. The benthos is mostly 
blanketed by an algal mat composed of a rich 
microflora assemblage (cyanobacteria, dia- 
toms, dinoflagellates) and macroalgae. Pre- 
liminary surveys in April 2005 showed that 
the fishpond bottom was 66% covered by 
A. spicifera, and that sand and oysters were 
the main algal substrate types. The alga was 
mostly absent on silt bottoms. Oysters are 
primarily found in shallow water (<1 m) and 
were therefore easy to reach for removal ef- 
forts. Oyster islands are present in 14% of 
the fishpond and are concentrated in the 
NW corner of Kaloko Fishpond. That area 
is where we concentrated our removal efforts. 

Measurement of Algal Coverage and Periodic 
Surveys 

It was difficult to get a good estimate of algal 
cover due to the three-dimensional structure 
of the plants, very low visibility (sometimes 
as low as 25 cm), and the daily movement of 
unattached drift algae. A 25 by 25 cm quadrat 
with nine, 1-cm 2 sampling squares evenly 
distributed was used to calculate "algal cover- 
age." Presence or absence of A. spicifera was 




Figure 1. Kaloko Fishpond and adjacent reef shelf with modifications of the seawall drawn in for an up-to-date 
image. The removal area (lined) in the northwest corner is the 2,200 m 2 removal and experimental area of this study. 
The lighter areas within the removal area are the oyster islands. Kaloko Fishpond is located in Kaloko-Honokohau 
National Historical Park just north of Kailua-Kona, on the west coast of Hawai'i Island.